The Cummins Generator Sizing Mistake That Cost Us $18,600 — and the Checklist I Use Now

September 2022: The order that looked simple

I handle generator procurement and spec review for a mid-size B2B distributor. I've been dealing with commercial generator orders for 9 years. I've personally made and documented six significant mistakes, totaling roughly $47,000 in wasted budget. Now I keep our team's checklist. This is the story of one of those mistakes.

In September 2022, we were buying standby power for three regional distribution centers. Each site needed backup power for lights, security, and HVAC. The load schedule said about 190 kW running. We had a Cummins generator catalogue, three quotes from commercial generator suppliers, and a deadline: commissioning in six weeks. I thought we were in good shape. That was the first mistake.

What I thought the Cummins generator catalogue told me

We looked at Cummins Generator models in the 250 kW range. The catalogue listed standby ratings, prime ratings, fuel consumption, and dimensions. The 250 kW standby diesel generator looked like the right fit. It was cheaper than the 350 kW option, and the electrical contractor's load sheet said 190 kW. That's a 31% margin. Honestly, that felt pretty safe.

But I made a classic procurement error: I treated the catalogue rating like a final answer. It isn't. A Cummins generator catalogue gives you baseline data. It doesn't tell you how that generator will perform at your site, with your loads, under your conditions.

The three things I didn't check

1. Starting kVA, not just running kW. The HVAC chillers had a locked rotor amps / starting surge that was way more than their running load. Our 190 kW running load could spike to almost 310 kW for a few seconds during chiller startup.
2. Altitude and temperature derating. One site was at about 1,200 ft elevation and saw 40°C summer days. Diesel generators lose capacity as altitude and temperature rise. The Cummins spec sheet had derating curves. I didn't read them carefully.
3. Application rating. Standby, prime, and continuous ratings are not interchangeable. ISO 8528 defines these categories. We bought a standby-rated unit, which was correct for emergency backup, but we were asking it to carry motor starting loads that the standby rating didn't cover in our conditions.

The commissioning day failure

Commissioning was in October 2022. The load bank test passed at 85% load. Everyone was happy. Then we transferred the building to generator power. The ATS closed, the generator picked up the load, and for a second it looked fine. Then the chiller started.

The generator went to about 110% load, the voltage dipped, and the main breaker tripped. The building went dark. We tried again. Same result. The Cummins generator wasn't broken. It was undersized for that specific starting event at that specific site.

That's when the costs started. We rented a 400 kW mobile diesel generator for 10 days: $9,800. We had to expedite a larger 350 kW unit: $6,400 price difference. Engineering review and re-commissioning: $2,400. Total damage: about $18,600, plus a 9-day delay. And a lot of embarrassment in front of our client.

I'm not an electrical engineer, so I can't tell you how to design protection or size conductors. What I can tell you from a procurement perspective is this: if you don't ask for starting kVA, derating, and application rating in writing, you're guessing.

What we did next

We replaced the 250 kW units with 350 kW Cummins Generator units at two sites, and added a soft starter at the third. The 350 kW units passed the load bank test at 100% and handled chiller startup without tripping. Even after I approved the replacement, I kept second-guessing. What if the chiller startup was still too much? I didn't relax until the full building transfer test passed twice in a row.

The upside of staying with 250 kW was about $8,000 in savings. The risk was failing commissioning. I kept asking myself: is $8,000 worth a 9-day delay and an angry client? The answer was obviously no. But in the moment, the lower quote looked so clean.

The backup generator specification guide I use now

After the third rejection in Q1 2024 — not this project, but a different spec issue — I created our pre-check list. We've caught 47 potential errors using it in the past 18 months. It's not fancy. It's just a shared digital checklist that forces us to document assumptions before we issue a PO.

• Define the application: standby, prime, or continuous. Confirm it against ISO 8528 rating definitions.
• Get the full load profile: running kW, running kVA, power factor, and starting kVA for every motor. Don't let anyone hand you a single number.
• Check site conditions: altitude, ambient temperature, fuel type, ventilation, and enclosure. Ask the commercial generator supplier for derating calculations in writing.
• Confirm code requirements: NFPA 110 for emergency standby systems, UL 2200 for stationary generator assemblies, and local AHJ requirements. For diesel units in the U.S., confirm EPA Tier 4 Final or applicable emissions tier.
• Review the Cummins generator catalogue correctly: use it to narrow models, then verify final sizing with a Cummins authorized dealer or a licensed engineer.
• Require a factory or dealer sizing report: if a supplier won't put sizing assumptions in writing, that's a red flag.
• Plan the commissioning test: load bank, transfer test, and real-load test. If the real load includes motor starting, test that exact sequence.

This worked for our three 250 kW to 350 kW standby applications. If you're sizing for a hospital, data center, or anything with life-safety loads, the calculus is different. I can only speak to our mid-size distribution centers with predictable load patterns. Your mileage may vary.

Bottom line

Efficiency isn't just about getting a quote faster. It's about reducing the chance that you'll pay for the same mistake twice. A shared checklist and a digital review log cut our spec review time from about 5 days to 2 days. That speed matters. But the real win is that we stopped treating the Cummins generator catalogue as a final sizing tool and started treating it as a starting point.

If you're buying bulk diesel generators or building a backup generator specification guide, don't skip the boring questions. Ask for starting kVA. Ask for derating. Ask which rating category you're buying. And get it in writing. That's the lesson I paid $18,600 to learn.